The invention relates to a combined guide fairlead and chain stopper for mooring chains on floating offshore installations.
On floating installations offshore the anchor chains are guided over guide fairleads mounted in the lower part of the hull. This arrangement generally has two objects, namely to move the point of attack for the mooring forces as far down on the installation as possible, in addition to guide the chain onto the anchor winches at the most favorable angle. The most usual type of guide fairlead for chains is a type of roller fairlead without chain stopper. This fairlead has a guide sheave of cast steel mounted in a cradle which pivots to both sides about a vertical axis. As a result, the chain may move in two planes towards the guide fairlead. The arrangement of a guide sheave results in the chain being treated in a gentle manner, but the arrangement consists of relatively many parts, thus making it quite heavy and expensive.
Another type of guide fairlead, the glide shoe fairlead (also called bending shoe fairlead), has a fixed glide track instead of a rotating guide sheave. It is simpler and less expensive than roller fairleads, but have clear drawbacks regarding how the chain is treated. Since the glide track is formed with an even radius in the longitudinal direction of the chain, the lying links will be subjected to bending moments when the chain is tensioned. This is the most undesirable form of loading for a chain, particularly since the bending moment occurs in the welding zone of the chain links. Since the chain is locked on deck, there is a relatively long length of chain extending between the chain stopper and the glide fairlead. This results in a certain gliding between the chain and the glide fairlead with varying tensile loads in the chain, the result being wear, particularly on the glide track, but also on the chain. There are glide shoe fairleads having an integrated chain stopper. One embodiment has the chain stopper mounted after the glide track in an extension thereof, so that the chain is fixed after the glide shoe fairlead and the chain hanging loosely along the glide track of the fairlead. In this case, the chain stopper is supported in bearings in two planes and may pivot with the chain. This arrangement transfers the wear from the chain to the supporting bearings of the chain stopper. A drawback of this arrangement is that it contains relatively many moving parts so that the manufacturing becomes expensive. It is also relatively difficult to operate the chain stopper from the deck of the platform.
The object of the present invention is to avoid the drawbacks mentioned above. This is obtained i.a. by the chain being fixed immediately above the glide track. The arrangement according to the invention, which is defined in claim 1, entails that the chain will lie in an even radius without being subjected to bending moments in the lying links.
The integrated chain stopper mounted immediately above the glide shoe entails that the chain is held fast without scrubbing in the longitudinal direction of the glide track, a fact that reduces wear on the chain and glide track to a minimum. Since the links of the chain will lie in the same position each time the chain is locked, the glide track may be formed with flat parts which support the lying links in their entire length. Between these flat parts the glide track is rounded in order for the chain to glide without problems when it is heaved in. In order to avoid that the links are subjected to high bending moments when the chain is heaved in, i.e. when they pass over the rounded parts, the bottom of the slot in the glide track is shaped in such a way that the standing chain links are pressed outwards and lift the lying chains somewhat up from the glide track. As a result, the chain will move in an approximately even radius when it is pulled in.
The glide shoe fairlead is made with side walls that continue for a relatively long distance after the bend in the glide track. This has the result that the fairlead will pivot along when the chain pivots laterally, even when the movements are small. If the glide fairlead had been provided with side walls of the glide track that were too short, it would have been standing still without pivoting along during smaller lateral movements of the chain. In such a case, the first chain links in the fairlead would have to absorb these movements, the result being that a few links would receive much wear. By transmitting this movement to the bearings in the vertical axis of the glide fairlead, the wear on the chain is reduced to a minimum.
A chain remaining in the same position in the longitudinal direction over a longer period of time may receive some wear on the links located in the glide fairlead. This results from vertical movements of the chain. In order to distribute this wear over several links of the chain, it is important to be able to pull in and let out the chain at regular intervals. The chain stopper is made in such a way that it can be remotely controlled from the deck. As a result, it does not take much work to pull in or let out the chain a few links at regular intervals.
The glide shoe fairlead according to the invention is shown in the appended drawings by way of a non-limiting exemplifying embodiment where: